1. Field of the Invention
This invention relates generally to transmission of signals, and, more particularly, to regulating transmission power of a signal carrying digital data.
2. Description of the Related Art
The telecommunications sector is undergoing a major metamorphosis. The change has been induced by three primary factors. First is the growing number of users demanding more bandwidth for faster data transmission. Second is the congestion in the Plain Old Telephone Service (POTS), designed for transmission of voice signals in analog form. Third is the Telecommunications Reform Act, which is fostering broader competition through deregulation. All three of the aforementioned factors call for a more effective and efficient means for transporting data at high speeds.
To meet the demand for high-speed communication, designers are seeking innovative and cost-effective solutions that take advantage of the existing network infrastructure. Several technological advancements have been made in the telecommunications industry that make use of the existing network of telephone wires. The most promising of these technologies is the Digital Subscriber Line (DSL) technology.
DSL is making the existing network of telephone lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and very high-speed data. The fact that more than six hundred million telephone lines exist around the world is a compelling reason that these lines will serve as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
There are now at least four popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Integrated Services Digital Network Digital Subscriber Line (IDSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, all four DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein.
DSL, as well as its later siblings, is making it possible for users to communicate over normal telephone wires at a much faster rate than before. The different flavors of DSL will converge around specific market niches and applications. For example, home users may favor ADSL for uses such as video-on-demand and Internet access. On the other hand, small businesses could find IDSL attractive for telecommuting and high-speed data transmission. Large businesses might choose VDSL to deliver Internet traffic or limited multimedia traffic to large businesses.
DSL technologies leave Plain Old Telephone Service undisturbed. Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels from 100 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 Megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS).
Although DSL and POTS systems can co-exist on one line (e.g., also referred to as "subscriber line"), the DSL traffic is not passed through the POTS circuitry due to the different bandwidth, voltage, and power needs between the two systems. The DSL signal is typically driven onto the subscriber line by a separate driver than is the POTS signal because the two signals are very different. A DSL signal has a lower voltage, wider bandwidth, and often requires a different number of bits of resolution when digitized.
The length of subscriber lines can vary considerably, ranging from a few thousand feet to over eighteen thousand feet. Generally, DSL line drivers are designed to operate over worst case scenarios. That is, DSL line drivers are typically designed to transmit signals over subscriber lines without regard to particular characteristics of each subscriber line, such as the actual length or the transmission quality of each specific subscriber line. In so doing, the DSL drivers are prone to use excessive power in transmitting signals on subscriber lines that may be shorter than the worst case. For example, at a DSL bandwidth of approximately 1 Megahertz, a DSL line driver will drive a signal onto a 1000 foot subscriber line with the same transmission power as it would if it were driving a signal onto a 10,000 foot subscriber line. Thus, what is needed is a method and apparatus for regulating transmission power of signals carrying digital data based on the characteristics of the transmission medium.
Designing a DSL driver to account for the worst case scenario not only wastes power on short lines but can also introduce excessive near end cross-talk (NEXT) in adjacent subscriber lines. Because the subscriber lines generally tend to be concentrated in the central switching office, any driver dissipating excess power can cause additional unwanted interference in the nearby subscriber lines.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.